The present invention relates to magnetic recording media such as magnetic tapes, and more specifically to surface treatment agents useful with metal oxide and ferromagnetic pigments in magnetic recording media.
Magnetic recording media are widely used in audio tapes, video tapes, computer tapes, disks and the like. Magnetic media may use thin metal layers as the recording layers, or may comprise particulate magnetic compounds as the recording layer. The latter type of recording media employs particulate materials such as ferromagnetic iron oxides, chromium oxides, ferromagnetic alloy powders and the like dispersed in binders and coated on a substrate. In general terms, magnetic recording media generally comprise a magnetic layer coated onto at least one side of a non-magnetic substrate (e.g., a film for magnetic recording tape applications).
In certain designs, the magnetic coating (or xe2x80x9cfront coatingxe2x80x9d) is formed as a single layer directly onto a non-magnetic substrate. In an effort to reduce the thickness of this magnetic recording layer, an alternative approach has been developed to form the front coating as a dual layer construction, including a support layer (or xe2x80x9clower layerxe2x80x9d) on the substrate and a reduced-thickness magnetic layer (or xe2x80x9cupper layerxe2x80x9d) formed directly on the support or lower layer. With this construction, the lower layer is typically non-magnetic or substantially non-magnetic, generally comprised of a non-magnetic powder and a binder. Conversely, the upper layer comprises a magnetic metal particle powder or pigment dispersed in a polymeric binder.
Finally, with magnetic recording tapes, a backside coating is typically applied to the opposing side of the non-magnetic substrate in order to improve the durability, conductivity, and tracking characteristics of the media.
The single layer of magnetic recording media, and both layers of dual layer magnetic recording media generally include a granular pigment. Popular pigments are metal oxides, ferromagnetic metal oxides, and ferromagnetic metal alloys; the material in the lower layer of the dual layer media is generally non-magnetic, and that in the upper layer is magnetic but similar pigment compounds may be used in both layers. Surface treatment agents are commonly used in magnetic media manufacturing to improve dispersion quality of particulate materials in both the magnetic recording layer and in the nonmagnetic lower layer. Typically the surface treatment agents are mixed with the pigment in a solvent, and then the surface treated pigments are combined with the other ingredients in a separate mixing step.
To be useful, a surface treatment agent must promote binder adsorption to the treated pigment surfaces. Different pigments have different surface properties, so different agents are required for different pigments. The agents vary depending on such factors as the basic or acidic properties of the pigment surface. Organic acids are the most commonly used surface treatment agent for magnetic materials with metal particles, as the metal particles often have a strongly basic surface. Recording media often utilize alpha hematite (xcex1-Fe2O3) particles in the formulations; dual layer recording media may utilize such particles in the nonmagnetic lower layer formulations, along with carbon black particles. The magnetic layer of such recording media often utilize gamma hematite (xcex3-Fe2O3) or ferromagnetic metal or metal alloy powders, along with carbon black particles.
All front coatings or layers of magnetic recording media generally include a binder composition. The binder composition performs such functions as dispersing the particulate materials, increasing adhesion between layers and to the substrate, improving gloss and the like. As might be expected, the formulation specifics associated with the requisite upper layer, lower layer, and back coat, as well as coating of the same to an appropriate substrate are highly complex, and vary from manufacturer to manufacturer; however, most binders include such materials as thermoplastic resins.
It has now been discovered that using certain low molecular weight, carboxylic acid containing azo dyes as surface treatment agents in one or more iron oxide or ferromagnetic powder-containing layers of a magnetic recording medium improves dispersion and modifies the rheology of the layer formulations, improving the gloss and the coating ability thereof
One aspect of the present invention relates to a magnetic recording medium including a non-magnetic substrate, a back coat, a lower support layer, and a magnetic upper layer. The substrate defines a front side and back side, with the back coat being formed on the back side. The lower layer is disposed over the front side of the substrate and includes a primary pigment material and a conductive carbon black material and a surface treatment agent for the primary pigment material dispersed in a binder.
Another aspect of the invention provides a surface treatment agent useful for iron oxide-containing non-magnetic lower layer of a dual layer magnetic recording medium. Specifically, the invention provides a surface treatment for xcex1-Fe2O3 containing lower layers comprising a low molecular weight azo dye having a formula:
xe2x80x94Arxe2x80x94Nxe2x95x90Nxe2x80x94Arxe2x80x94
wherein Ar represents an aryl group, each of which comprises at least one pendant group selected from an electron donor, and an electron acceptor.
Additionally, the invention provides a dual layer magnetic recording medium comprising a recording layer, and a nonmagnetic lower layer wherein the lower layer comprises a multiplicity of Fe2O3 particles, and as a surface treatment therefore, a low molecular weight azo-dye comprising the general formula:
xe2x80x94Arxe2x80x94Nxe2x95x90Nxe2x80x94Arxe2x80x94
wherein Ar represents an aryl group, each of which comprises at least one pendant group selected from an electron donor, and an electron acceptor.
Another aspect of the invention provides a single layer magnetic recording medium comprising a non-magnetic substrate having a front side and a back side, a back coat formed on the back side, and a magnetic recording layer formed on the front side, wherein said magnetic layer comprises a ferromagnetic powder dispersed in a binder, and a surface treatment agent comprising a low molecular weight azo dye.
Another aspect of the invention provides a magnetic medium comprising a non-magnetic substrate having a front side and a back side, a back coat formed on the back side, and a magnetic recording layer formed on the front side, wherein the back coat comprises an alpha iron oxide dispersed in a binder, and a low molecular weight azo dye having the following general formula:
xe2x80x94Arxe2x80x94Nxe2x95x90Nxe2x80x94Arxe2x80x94
wherein Ar represents an aryl group, each of which independently comprises at least one pendant group selected from an electron donor, and an electron acceptor.
In one preferred embodiment, the surface treatment agent is an acidic form of Chrome Orange.
As used herein, all weights, ratios and amounts are by weight unless otherwise specified.
As used herein, the following terms have these meanings:
1. The term xe2x80x9clow molecular weightxe2x80x9d means having a molecular weight of less than about 500.
2. The term xe2x80x9ccoercivityxe2x80x9d means the intensity of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero after it has reached saturation.
3. The term xe2x80x9csoftxe2x80x9d means having a coercivity of less than 300 Oe, or a Moh""s hardness of less than 5.